Engine exhaust gases may be highly correlated with engine air-fuel ratio. For example, combustion of richer air-fuel mixtures in an engine may lead to higher HC and CO emissions while leaner mixtures may lead to higher NOx emissions. Engine exhaust gases may be directed to a catalyst where they are processed into more desirable compounds such as H2O and CO2. However, if engine exhaust gases are not rich or lean as expected due to engine air-fuel ratio variation between an engine's cylinders, engine emissions may degrade.
One way to determine and correct air-fuel ratio variation between engine cylinders is to sense engine exhaust gases via an oxygen sensor. However, the oxygen sensor may be exposed to exhaust gases that are a combination of gases from different engine cylinders. Therefore, it may be difficult to accurately determine air-fuel variations between different engine cylinders. Further, engine exhaust system geometry for cylinders having a large number of cylinders may bias sensor readings toward output of one cylinder more than other cylinders. Consequently, it may be even more difficult to determine air-fuel imbalance for engines having more than a few cylinders.
The inventors herein have recognized the above-mentioned limitations and have developed a method for detecting cylinder air-fuel imbalance that is not subject to exhaust system geometry and that may signal to noise ratio for determining cylinder to cylinder air-fuel imbalance. The method comprises: during a deceleration fuel shut-off (DFSO) event where all cylinders of an engine are deactivated, selectively sequentially combusting air and fuel in cylinders of a cylinder group in the engine, each cylinder fueled via a fuel pulse width, and adjusting fuel injected to one or more cylinders in the cylinder group in response to variation of engine torque from an expected engine torque during the DFSO event.
By selectively activating cylinders during DFSO and determining engine torque, it may be possible to provide the technical result of improving cylinder to cylinder air-fuel ratio imbalance detection and correction. For example, torque produced via a cylinder may be inferred from engine acceleration at a time when other engine cylinders are deactivated so that torque output from one cylinder is not intermingled with torque produced via a cylinder adjacent to the one cylinder in a combustion order of the engine. In this way, an estimate of torque produced by the cylinder may be improved as compared to if engine torque were determined in the presence of other activated cylinders. The improved engine torque estimate may be compared to an expected engine torque estimate to determine an air-fuel correction factor for adjusting the cylinder's air-fuel ratio. Thus, it may be possible to correct an engine's cylinder to cylinder air-fuel ratio variation without the engine's exhaust system geometry biasing cylinder to cylinder air-fuel ratio imbalance estimates. Further, by determining torque of an activated cylinder when adjacent cylinders in the engine's firing order are deactivated, it may be possible to improve an estimate of torque produced by a cylinder which is a basis for determining cylinder air-fuel variation.
The present description may provide several advantages. For example, the approach may improve cylinder to cylinder air-fuel imbalance estimation for engines having oxygen sensor placement that may be influenced by cylinder air-fuel observations. Further, the approach may provide an improved signal to noise ratio of air-fuel variation for engines having greater numbers of cylinders by preventing combustion in cylinders that are adjacent to a cylinder being evaluated for torque production. Further still, the approach may be provided during engine operating conditions where the approach is less likely to be sensed via a vehicle operator.
The above discussion includes recognitions made by the inventors and not admitted to be generally known. It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.